Study shows how droplets can “levitate” on liquid surfaces

A drop or two of cold cream in hot coffee can go a long way toward improving one’s morning. But what if the two liquids didn’t mix?

MIT scientists have now explained why under certain conditions a droplet of liquid should not coalesce with the liquid surface below. If the droplet is very cold, and the bath sufficiently hot, then the droplet should “levitate” on the bath’s surface, as a result of the flows induced by the temperature difference.

The team’s results, published today in the Journal of Fluid Mechanics, offer a detailed, mathematical understanding of drop coalescence, which can be observed in everday phenomena, from milk poured in coffee to raindrops skittering across puddles, and sprays created in surf zones.

The results may help researchers understand how biological or chemical substances are spread by rain or other sprays in nature. They could also serve as a guide for droplet-based designs, such as in microfluidic chips, in which droplets carrying various reagents can be designed to mix only in certain locations in a chip, at certain temperatures. With this new understanding, researchers could also engineer droplets to act as mechanical ball bearings in zero-gravity environments.

“Based on our new theory, engineers can determine what is the initial critical temperature difference they need to maintain two drops separately, and what is the maximum weight that a bearing constructed from these levitating drops would be able to sustain,” says Michela Geri, a graduate student in MIT’s Department of Mechanical Engineering and the study’s lead author. “If you have a fundamental understanding, you can start designing things the way you want them to work.”

Geri’s co-authors are Bavand Keshavarz, a lecturer in mechanical engineering, John Bush, professor of applied mathematics in MIT’s Department of Mathematics, and Gareth McKinley, the School of Engineering Professor of Teaching Innovation. Courtesy: MIT News